Raman spectroscopy is similar to infrared (IR), including near infrared (NIR), spectroscopy but has several advantages. The Raman effect is highly sensitive to slight differences in chemical composition and crystallographic structure. These characteristics make it very useful for substance identification such as the investigation of illegal drugs as it enables distinguishing between legal and illicit compounds, even when the compounds have a similar elemental composition. In other applications, taggants, with known Raman signatures, are used as markers for goods.
Raman spectroscopy has additional advantages. When using IR spectroscopy on aqueous samples, a large proportion of the vibrational spectrum can be masked by the intense water signal. In contrast, with Raman spectroscopy, aqueous samples can be more readily analyzed since the Raman signature from water is relatively weak. Also, because of the poor water signature, Raman spectroscopy is often useful when analyzing biological and inorganic systems, and in studies dealing with water pollution problems.
Raman scattering may be regarded as an inelastic collision of an incident photon with a molecule. The photon may be scattered elastically, that is without any change in its wavelength, and this is known as Rayleigh scattering. Conversely the photon may be scattered inelastically resulting in the Raman effect.
There are two types of Raman transitions. Upon collision with a molecule, a photon may lose some of its energy. This is known as Stokes radiation. Or, the photon may gain some energy—this is known as anti-Stokes radiation. This happens when the incident photon is scattered by a vibrationally excited molecule—there is gain in energy and the scattered photon has a higher frequency or shorter wavelength.
When viewed with a spectrometer, both the Stokes and anti-Stokes radiation are composed of lines that correspond to molecular vibrations of the substance under investigation. Each compound has its own unique Raman spectrum, which can be used as a fingerprint for identification.
The Raman process is nonlinear. When incident photons have a low intensity, only spontaneous Raman scattering will occur. As the intensity of the incident light wave is increased, an enhancement of the scattered Raman field can occur in which initially scattered Stokes photons can promote further scattering of additional incident photons. With this process, the Stokes field grows exponentially and is known as stimulated Raman scattering (SRS).
One disadvantage associated with Raman spectroscopy, however, is fluorescence from the sample or impurities in the sample. In many cases, the fluorescence response can overwhelm the typically much weaker Raman signature. This can make detection of small peaks in the Raman signature difficult. Often, fluorescence can be mitigated by moving to a longer wavelength excitation. This can create other problems, however.
One robust solution to the fluorescence response is using excitation signals at multiple wavelengths. Specifically, in the past others have suggested to use excitation signals that comprise two excitation wavelengths, generated by two different single frequency lasers. This is sometimes referred to as Shifted Excitation Raman Difference Spectroscopy (SERDS). Then, by looking at the spectrums generated by each of the wavelengths, the fluorescence signal can be identified since it changes very little with excitation wavelength, whereas the Raman signal changes as a direct function of the excitation wavelength. In the simplest example, the spectra at two excitation wavelengths are subtracted to remove the highly stationary fluorescence response. Recently, this solution has been further enhanced by using a continuously tunable semiconductor diode laser system. In these systems, the spectral response of the sample is monitored as the excitation signals wavelength is scanned over a scan range. By looking at how the spectral response changes with the tuning of the excitation signal and how it does not change, the Raman response can be separated from the fluorescence response of the sample.